1. Field of the Invention
The present invention generally relates to multi-piece golf balls having a solid core of at least one layer and cover of at least one layer. At least one of the core layers comprises a thermoplastic ionomer composition and at least one of the core layers comprises a thermoplastic polyamide composition. Preferably, the ball contains a dual-core having an inner core and surrounding outer core layer or a multi-layered core having an inner core, intermediate core layer, and outer core layer.
2. Brief Review of the Related Art
Multi-piece, solid golf balls are used today by recreational and professional golfers. Basically, these golf balls contain an inner core protected by a durable cover. The core and cover may be single or multi-layered. For example, three-piece golf balls having an inner core, inner cover layer, and outer cover layer may be used. In other instances, golfers will use a four-piece ball containing a dual-core (inner core and surrounding outer-core layer) and dual-cover (inner cover layer and surrounding outer cover layer). Intermediate (casing) layer(s) may be disposed between the core and cover layers to impart various properties and thus form five-piece balls and the like. Normally, the core layers are made of a natural or synthetic rubber material such as, for example, styrene butadiene, polybutadiene, poly(cis-isoprene), or poly(trans-isoprene); and the cover layers are made of a durable material such as, for example, ethylene acid copolymer ionomer resins, polyamides, polyesters, polyurethanes, or polyureas. Today, the industry is interested, among other things, in making balls that can rebound faster, retain more total energy when struck with a club, and have longer flight distance.
In general, the rebounding performance of a golf ball is based on its initial velocity after being struck by the face of a golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “coefficient of restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under these conditions. Balls with a higher rebound velocity have a higher COR value. To improve the resiliency and rebounding performance of the golf ball, the industry has focused primarily on the material and construction of the ball's core.
Golf balls having single-layered and multi-layered cores have been developed. For example, Bulpett et al, US Patent Application Publication US 2009/0227394 discloses multi-layered core construction comprising: a) an inner core formed from a first thermoset rubber composition; b) an intermediate core layer formed from a partially-neutralized or highly-neutralized ionomer composition; and c) an outer core formed from a second thermoset rubber composition. A cover layer having a thickness of about 0.01 to 0.05 inches and a surface hardness of about 60 Shore D or less is formed around the core.
Sullivan et al., US Patent Application Publication No. US 2009/0017940 discloses golf balls having a dual-core and a single-layered cover. The dual-core includes an inner core formed from a rubber composition and an outer core layer formed from a highly neutralized polymer (HNP) composition comprising an ethylene acid copolymer. In the HNP composition, at least 80% of all acid groups are neutralized. The inner core has an outer surface hardness of less than 80 Shore C; the outer core layer has an outer surface hardness of 56 Shore D or greater; and the cover layer has a material hardness of 60 Shore D or less.
Sullivan et al., U.S. Pat. Nos. 7,357,736 and 7,211,008 disclose golf balls comprising: a) an inner core layer formed from a diene rubber composition; (b) an outer core layer formed from a high modulus highly neutralized polymer (HNP) comprising a highly neutralized ethylene/(meth)acrylic acid copolymer having a modulus of from 45,000 psi to 150,000 psi; (c) an intermediate core layer disposed between the inner core layer and the outer core layer and formed from a low modulus HNP composition comprising a highly neutralized ethylene/(meth)acrylic acid/alkyl (meth)acrylate copolymer having a modulus of from 1,000 psi to 50,000 psi. In the HNP compositions, at least 80% of all acid groups are neutralized.
Nesbitt et al., U.S. Pat. No. 7,147,578 discloses golf balls containing a dual-core. The inner core (center) and outer core layer may be formed from a thermoset material or a thermoplastic material. The '578 patent discloses suitable thermoset materials as including polybutadiene or any natural or synthetic elastomer, metallocene polyolefins, polyurethanes, silicones, polyamides, and polyureas. Suitable thermoplastic materials are described as including ionomers, polyurethane elastomers, and combinations thereof. In particular, the '578 patent discloses the following commercially available thermoplastic resins are preferred for use in the dual-cores: Capron® 8351 (Allied Signal Plastics), Lexan® ML5776 (General Electric), Pebax® 3533 (Elf Atochem), and Hytrel® G4074 (DuPont).
As noted above, ethylene acid copolymer ionomers have been used to form components in golf balls. In general, polyamide compositions also have been used in golf ball constructions. For example, Kim et al, US Patent Application Publication US 2010/0167845 discloses golf balls having components including intermediate layers and outer cover layers prepared from blends of polyamides mixed with one or more functional polymer modifiers. The functional polymer modifier incorporates a copolymer or a terpolymer having a glycidyl group, hydroxyl group, maleic anhydride group or carboxylic group. According to the '845 Publication, when these materials are used in golf ball covers or intermediate layers, the balls exhibit improved impact durability.
Rajagopalan et al., U.S. Pat. No. 6,800,690 discloses a golf ball comprising a core and a cover, wherein at least a portion of the cover is formed from a composition comprising a polyamide copolymer formed from reactants consisting essentially of at least one dibasic acid and at least one diamine, wherein the at least one dibasic acid comprises about 14 carbons to about 40 carbons, and wherein the polyamide copolymer lacks affinity for water.
Bellinger and Sullivan, U.S. Pat. No. 5,886,103 discloses a golf ball having a cover layer made from a resin comprising a polyamide component, an ionomeric component, and an ester component comprising a copolymer of ethylene and ethylacrylate, wherein the content of the polyamide component is at least 23 wt % of the resin composition. The golf ball has a coefficient of restitution of at least 0.750.
Although some conventional multi-layered core constructions are generally effective in providing high resiliency golf balls, there is a continuing need for improved core constructions in golf balls. Particularly, it would be desirable to have multi-layered core constructions that could be manufactured efficiently, have relatively low material costs, and provide the ball with high quality and performance properties. In particular, the core should provide the ball with good flight distance along with a comfortable and soft feel. The present invention provides core constructions and resulting golf balls having such properties as well as other advantageous features, and benefits.